Electronic parts which include a resistive or dielectric substrate coated with a thin layer of conductive metal find important use in industry. The conductive metal layer is applied to the substrate by spraying, brushing, dipping, rolling or screen printing a coating composition of finely divided, metal particles suspended in a liquid vehicle. Spraying is a preferred method of applying the conductive metal layer because it is fast and it permits laying down uniform, thin layers on very intricately shaped parts. Subsequently, the coating composition is dried at ambient or slightly higher temperature, then the part is fired at very high temperature to permanently bond the metal to the substrate.
Traditionally, the solvent in the liquid vehicle for silver coating compositions has been primarily a volatile organic compound, (VOC). A VOC-based vehicle has several drawbacks. It may be toxic, flammable, and possibly explosive. The VOC raw material is costly to purchase and the waste is expensive to discard. Generally, VOC solvent emitted from coating operations is an environmental air pollutant. Expensive equipment and procedures are required to capture and contain VOC solvent emissions in order to reduce air pollution and enable proper waste disposal.
While it is desirable to have a sprayable, silver coating composition based on an aqueous vehicle, heretofore it has been unknown how to formulate such a composition. Lubricant on the silver is one obstacle to achieving aqueous based coating compositions. Silver flake is usually made by milling lubricated silver powder. The lubricant is very important for assuring that the metal particles form into flakes as a result of the milling. It has been found that residual lubricant on the flake surface can make the flake hydrophobic and difficult to disperse in water. Poorly dispersed silver flake will not spray uniformly, and produces an irregular coating on the substrate. Furthermore, it is common in the industry to use very small hole spray nozzles. Poorly dispersed silver coating composition can frequently clog small hole nozzles. It is also desirable to have a very high concentration of silver particles in the composition in order to lay down at high speed a specified coating thickness in the least number of spraying applications, and preferably only one application. However, if silver particle concentration is too high, the coating composition viscosity rises excessively and adversely affects spraying performance. High silver concentration also increases the tendency of the silver to settle and irreversibly agglomerate. Prior to this invention, a low viscosity, highly silver-loaded, sprayable coating composition based on an aqueous vehicle was unavailable.
Accordingly, it is a main objective of this invention to provide for the safe, environmentally benign production of silver-coated, ceramic parts. It is a feature of this invention that a silver coating composition contains an aqueous liquid vehicle which is substantially free of volatile organic compounds. The present invention advantageously allows a silver coating composition to be coated on ceramic parts without the need to recover and safely dispose of large quantities of volatile organic compounds.
It is another objective of the present invention to provide a silver coating composition that can be applied using conventional coating equipment and without changing existing manufacturing procedures significantly. It is an advantage of this invention that well-dispersed, aqueous coating compositions based on lubricated silver flake can be formulated. It is a feature of this invention that the coating compositions have an optimum combination of low viscosity and very high loading of silver flake. Therefore it is another advantage of the present invention that silver coating compositions can be applied in only a single, high speed spray application using existing, small hole, spray nozzles to provide a very uniform, firmly attached, metal coating.
It is still another objective of the present invention to provide a stable coating composition in which the silver flake resists separation from the liquid vehicle on standing in storage. The present invention includes a combination of ingredients which promotes and maintains uniform dispersion of the silver in the aqueous-based vehicle. It is an additional feature of this invention that any solids which settle can be readily redispersed in the vehicle with minimum agitation.
Accordingly, there is presently provided a coating composition comprising:
(a) about 30 to about 80 wt % silver flake coated with a lubricant comprising at least one straight-chain organic acid or salt of said acid, said lubricant having from 6 to 18 carbon atoms; PA1 (b) about 1.5 to about 4.0 wt % substantially completely water soluble polymer binder; PA1 (c) about 0.5 to about 8.0 wt % substantially completely water soluble co-solvent; and PA1 (d) a solvent effective amount of water; PA1 wherein said percentages are based on total weight of the coating composition. PA1 (1) spraying a coating composition onto said substrate to form a coated part, said coating composition comprising: PA1 wherein said percentages are based on total weight of the coating composition; and PA1 (2) drying said coated part at temperature in the range of about 20.degree. C. to about 200.degree. C. until at least 95% of water in the coating composition is removed.
There is also provided a process of coating a resistive or dielectric substrate with a layer of conductive metal, comprising the steps of:
(a) about 30 to about 80 wt % silver flake coated with a lubricant comprising at least one straight-chain organic acid or salt of said acid, said lubricant having from 6 to 18 carbon atoms; PA2 (b) about 1.5 to about 4.0 wt % substantially completely water soluble polymer binder; PA2 (c) about 0.5 to about 8.0 wt % substantially completely water soluble co-solvent; and PA2 (d) a solvent effective amount of water;
When it is desired to permanently attach the silver layer to a suitably temperature resistant substrate, such as ceramic, the composition can contain an optional, sintering adhesive component and the coated part can be fired at a temperature from about 600.degree. C. to about 950.degree. C. for a duration sufficient to activate the sintering adhesive. Subsequently, the part is cooled to solidify the sintering adhesive.